In efforts to detect retrovirus nucleotide sequences in human DNA, the squirrel monkey retrovirus (SMRV) was molecularly cloned in a bacteriophage vector. The cloned SMRV DNA was biologically active as demonstrated by DNA transfection assays. A detailed restriction enzyme map of the viral genome was constructed. Using this information, discrete fragments of SMRV DNA were prepared by subcloning in a plasmid vector. These subcloned DNA fragments, along with the genomic viral DNA, are being used as probes to detect homologous sequences in human DNA and also to study the relationship between SMRV and other retroviruses.